1. Field of the Invention
The invention relates to an apparatus for polishing a substrate for planarization by chemical mechanical polishing. The invention relates further to a method of chemical mechanical polishing.
2. Description of the Related Art
FIGS. 1A to 1E illustrate respective steps in a method of forming a buried metal layer in a semiconductor device.
First, as illustrated in FIG. 1A, a semiconductor substrate 101 including active devices fabricated thereon is covered entirely with an insulating film 102.
Then, a resist film 105 having a certain pattern is formed on the insulating film 102, and subsequently, the insulating film 102 is etched with the patterned resist film 105 being used as a mask, to thereby form a contact hole 106 through the insulating film 102, as illustrated in FIG. 1B.
After removal of the resist film 105, as illustrated in FIG. 1C, a barrier film 103 composed of metal such as Ti or Ta is deposited over the insulating film 102 so that the contact hole 106 is covered at a sidewall and a bottom thereof with the barrier film 103.
Then, as illustrated in FIG. 1D, an electrically conductive layer 104 is deposited over the product to thereby fill the contact hole 106 with the electrically conductive layer 104.
Then, the electrically conductive film 104 is planarized by means of a chemical mechanical polishing apparatus 107, as illustrated in FIG. 1E. Thus, a buried metal layer 108 is formed.
The chemical mechanical polishing apparatus 107 includes a carrier on which a wafer to be polished is fixed, and a rotatable level block on which a polishing pad is mounted. A wafer is compressed onto a rotating polishing pad to thereby be polished. While a wafer is being polished by the polishing pad, polishing powder such as alumina or silica, and polishing slurry containing etchant such as H2O2 are supplied between the polishing pad and the wafer.
FIG. 2 illustrates a conventional apparatus for polishing a wafer by chemical mechanical polishing. The illustrated apparatus is comprised of a level block 23 connected to a rotatable shaft 24, a polishing pad 29 fixed onto the level block 23, a wafer holder 26 connected to a rotatable shaft 27 and holding a wafer 25 on a bottom thereof, and a slurry source 30 supplying polishing slurry onto the polishing pad 29 through a slurry supply port 21.
The wafer 25 is sandwiched between the polishing pad 29 and the wafer holder 26. While the wafer 25 is being polished by the polishing pad 29, polishing slurry 22 is supplied between the polishing pad 29 and the wafer 25 around a periphery of the wafer 25.
Though the illustrated apparatus is designed to have one wafer holder 26, the apparatus may be designed to have a plurality of wafer holders 26. For instance, the apparatus may be designed to have four wafer holders 26 equally spaced from one another above the level block 23 in order to concurrently polish four wafers at a time.
A conventional apparatus for polishing a wafer, such as the apparatus illustrated in FIG. 2, is accompanied with a problem of non-uniformity in polishing speed in a wafer, which results in that a wafer is polished around a center thereof to a greater degree than a periphery thereof.
In order to overcome this problem, there has been suggested a first polishing apparatus in which a polishing pad mounted on a level block is formed with a plurality of small through-holes through which polishing slurry is supplied onto a surface of the polishing pad from a polishing slurry source. The small through-holes are positioned in concentration with an axis of the polishing pad 29. Since polishing slurry is uniformly supplied between a wafer and the polishing pad, it would be possible to keep a polishing speed constant to thereby enhance uniformity in polishing a wafer.
There has been suggested also a second polishing apparatus in which a polishing pad is composed of porous material in order to enhance uniformity in polishing a wafer.
However, since a wafer having a greater diameter is compressed onto a polishing pad at a greater pressure around a center thereof than a periphery thereof, a polished wafer would have a cross-section like a cross-section of a concave lens, if a wafer is polished in accordance with the above-mentioned first or second polishing apparatuses in which polishing slurry is uniformly supplied to a surface of a wafer, whereas a polished wafer would have a cross-section like a convex lens, if a wafer is polished in accordance with the apparatus illustrated in FIG. 2.
In order to avoid this problem, Japanese Unexamined Patent Publication No. 5-13389 has suggested a polishing apparatus which has the same structure as that of the above-mentioned first and second polishing apparatus, but is capable of controlling an amount of polishing slurry at a predetermined position of a polishing pad for the purpose of enhancing uniformity in polishing a wafer.
Specifically, the suggested polishing apparatus is formed with a plurality of through-holes through which polishing slurry is supplied onto a surface of a polishing pad, in such a manner that the number of through-holes per a unit area in a region closer to a center of a polishing pad is designed to be greater than the number of through-holes per a unit area in a region closer to a periphery of a polishing pad, or that a through-hole located closer to a center of a polishing pad is designed to have a greater diameter than a diameter of a through-hole located closer to a periphery of a polishing pad.
A diameter of a wafer necessary to be polished is increasing. For instance, a diameter of a wafer to be polished years ago was 6 inches (about 15 cm), but a diameter of a wafer to be polished presently is in the range of 8 to 10 inches (about 20 to about 25 cm). Such a wafer having a great diameter could not be polished by means of such an apparatus as illustrated in FIG. 2, because the level block 23 has to have too much area, which results in too high load to the apparatus.
Hence, there has been suggested such a polishing apparatus as illustrated in FIG. 3A, in order to avoid the above-mentioned problem. The illustrated apparatus is comprised of a rotatable carrier 2 supporting a wafer 1 at a bottom thereof, a level block 3, a polishing pad 4 mounted on the level block 3 and positioned in facing relation to the carrier 2, and a motor 5 for rotating the level block 3 around a rotation axis. The polishing pad 4 is formed with a plurality of through-holes equally spaced from one another.
The wafer 2 is made to rotate, and then, is compressed onto the rotating polishing pad 4. While the wafer 1 is being polished. While the wafer 2 is being polished, slurry 6 is supplied onto a surface of the polishing pad 4 through the through-holes.
In order to enhance uniformity in polishing the wafer 1, the level block 3 is rotated by means of the motor 5 in such a manner that the rotation axis of the level block 3 moves along an arcuate path. That is, the level block 3 makes so-called orbital revolution.
FIG. 4 shows a positional relation in orbital revolution between the wafer 1 rotating around a rotation axis A and the polishing pad 4 rotating around a rotation axis B. As illustrated in FIG. 4, if viewed from the rotation axis A, the rotation axis B rotates around the rotation axis A.
As mentioned earlier, if a wafer is polished with polishing slurry being supplied onto a surface of a polishing pad through through-holes formed with the polishing pad, there is caused a problem that a wafer is polished to a greater degree in a central region than in a peripheral region, resulting in that a wafer is concave in a central region thereof. If a wafer is non-uniformly polished as mentioned above, an electrically conductive film such as the electrically conductive film 104 illustrated in FIG. 1D partially remains non-removed on an insulating film such as the insulating film 102, resulting in current leakage between wirings.
In order to avoid such a problem, it is necessary to sufficiently polish a wafer. However, this may result in that a wiring to be formed on an insulating film has different heights above a central region and a peripheral region of a wafer. Accordingly, a wiring resistance above a central region of a wafer becomes different from a wiring resistance above a peripheral region of a wafer with the result of deterioration in electro-migration (EM).
It is an object of the present invention to provide an apparatus for polishing a wafer, which apparatus is capable of enhancing uniformity in polishing. It is also an object of the present invention to provide a method of doing the same.
The inventors had conducted a lot of experiments in order to accomplish the above-mentioned object, and had found out that if a polishing pad is designed to include a region where there are formed no through-holes through which polishing slurry is supplied to a surface of the polishing pad, it would be possible to enhance uniformity in polishing a wafer.
Specifically, in one aspect of the invention, there is provided an apparatus for polishing a substrate, including (a) a polishing pad formed with a plurality of through-holes through which polishing material is supplied to a surface of the polishing pad, (b) a level block on which the polishing pad is mounted, and (c) a rotatable carrier for supporting a substrate thereon, the carrier being positioned in facing relation with the level block, the level block being rotatable around a rotation axis thereof with the rotation axis being moved along an arcuate path, and causing the polishing pad to make contact with the substrate for polishing the substrate, the polishing pad having a first ring-shaped region concentric thereto where no through-holes are formed.
It is preferable that the first ring-shaped region has a width equal to or greater than 10%, more preferably 20%, of a radius of the polishing pad.
It is preferable that the through-holes are positioned in alignment with a peripheral region of the substrate when an axis of the level block is in alignment with an axis of the carrier.
It is preferable that the through-holes are positioned in a second ring-shaped region having an outer periphery common to an outer periphery of the polishing pad and having a width equal to 5% or smaller of a radius of the polishing pad.
It is preferable that the polishing pad includes a circular region concentric to the polishing pad and located inside the first ring-shaped region, and a third ring-shaped region located outside the first ring-shaped region, the circular region and the third ring-shaped region including the through-holes therein. In this arrangement, it is preferable that the third ring-shaped region has an outer periphery common to an outer periphery of the polishing pad. It is also preferable that the through-holes formed in the third ring-shaped region are positioned in alignment with a peripheral region of the substrate when an axis of the level block is in alignment with an axis of the carrier.
It is preferable that a total area of the through-holes varies in a radius-wise direction of the, polishing pad. For instance, the number of the through-holes per a unit area may be designed to decrease in a direction from an outer periphery to a center of the polishing pad. As an alternative, diameters of the through-holes may be designed to decrease in a direction from an outer periphery to a center of the polishing pad.
There is further provided an apparatus for polishing a substrate, including (a) a polishing pad formed with a plurality of through-holes through which polishing material is supplied to a surface of the polishing pad, (b) a level block on which the polishing pad is mounted, and (c) a rotatable carrier for supporting a substrate thereon, the carrier being positioned in facing relation with the level block, the level block being rotatable around a rotation axis thereof with the rotation axis being moved along an arcuate path, and causing the polishing pad to make contact with the substrate for polishing the substrate, the polishing pad having a circular region concentric thereto where no through-holes are formed.
It is preferable that the circular region has a radius equal to or smaller than 95% of a radius of the polishing pad.
It is preferable that the circular region has a radius equal to or greater than 30% of a radius of the polishing pad.
In another aspect of the present invention, there is provided a method of carrying out chemical mechanical polishing to a substrate, including the steps of (a) rotating a level block on which a polishing pad is mounted, relative to a carrier on which a substrate is mounted, around a rotation axis thereof with the rotation axis being moved along an arcuate path, and (b) supplying polishing material on a surface of the polishing pad while the substrate is being polished by the polishing pad, in a region other than a first ring-shaped region concentric to the polishing pad.
For instance, the polishing material may be supplied on a surface of the polishing pad through through-holes formed with the polishing pad.
It is preferable that the polishing material is supplied on a surface of the polishing pad in a second ring-shaped region having an outer periphery common to an outer periphery of the polishing pad and having a width equal to 5% or smaller of a radius of the polishing pad.
It is preferable that the polishing pad includes a circular region concentric to the polishing pad and located inside the first ring-shaped region, and a third ring-shaped region located outside the first ring-shaped region, the polishing material being supplied into the circular region and the third ring-shaped region.
It is preferable that the polishing material is supplied onto a surface of the polishing pad in a varying amount in a radius-wise direction of the polishing pad. For instance, the polishing material may be supplied in a greater amount in a region closer to a center of the polishing pad.
There is further provided a method of carrying out chemical mechanical polishing to a substrate, including the steps of (a) rotating a level block on which a polishing pad is mounted, relative to a carrier on which a substrate is mounted, around a rotation axis thereof with the rotation axis being moved along an arcuate path, and (b) supplying polishing material on a surface of the polishing pad while the substrate is being polished by the polishing pad, in a region other than a circular region concentric to the polishing pad.
It is preferable that the circular region has a radius equal to or smaller than 95% of a radius of the polishing pad.
It is preferable that the circular region has a radius equal to or greater than 30% of a radius of the polishing pad.
In the apparatus in accordance with the present invention, a polishing pad is designed to have a region in which through-holes through which polishing material is supplied to a surface of the polishing pad are not formed. In the method in accordance with the present invention, polishing material is supplied to a surface of a polishing pad in a region other than a certain region of the polishing pad. As a result, the present invention makes it possible to accomplish uniformity in polishing rate in a high degree. Hence, when a buried metal layer is to be formed by chemical mechanical polishing, a resultant semiconductor device could have superior resistance to electro-migration (EM).
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.